Speaker of the House Nancy Pelosi, D-Calif., arrives at the Capitol in Washington on Friday. J. Scott Applewhite/Associated Press

WASHINGTON — Speaker Nancy Pelosi said the House will take steps next week to transmit the articles of impeachment against President Trump, ending a three-week standoff but confronting the Senate with only the third trial in U.S. history to remove a chief executive.

In a letter to her Democratic colleagues, Pelosi said Friday she was proud of their ”courage and patriotism” and warned that senators now have a choice as they consider the charges of abuse and obstruction against the president.

“In an impeachment trial, every Senator takes an oath to do ‘impartial justice according to the Constitution and laws,”’ Pelosi wrote. “Every Senator now faces a choice: to be loyal to the President or the Constitution.”

The trial could begin next week. The Constitution gives the House the sole power to impeach a president, but the Senate the ability to render a verdict when it convenes as the Court of Impeachment.

Pelosi was particularly upbeat Friday as she strode through the Capitol, despite the mounting pressure on her to quit delaying the trial. Her decision to end the showdown with Senate Majority Leader Mitch McConnell does not fully bring closure to the question of whether the Senate will consider new witnesses, as some want, shifting pressure on senators to decide.

Trump swiftly signaled his intention of blocking any testimony from John Bolton, the brash former national security adviser who could be a wildcard witness in the trial. Bolton has said he would appear before the Senate if he received a subpoena.

At the same time, a key centrist, Sen. Susan Collins of Maine, whose vote is among those most watched, announced Friday she was in discussions with other Republicans on a strategy that would allow the Senate to hear new testimony.

While the rules of Senate trial remain unsettled, the outcome is not. Trump is widely expected to be acquitted of the charges that he abused power by pressuring Ukraine to investigate Joe Biden, then obstructed Congress in its investigation. No president has ever been removed by the Senate.

“Ridiculous,” Trump told Fox News’ Laura Ingraham about the speaker’s gambit. “Nancy Pelosi will go down as the least successful speaker of the House in the history of our nation,” he said.

Rep. Liz Cheney, R-Wyo., speaks with reporters as lawmakers leave the Capitol in Washington on Friday. House Speaker Nancy Pelosi said the House will take steps next week to send articles of impeachment to the Senate for President Trump’s trial. J. Scott Applewhite/Associated Press

Asked if he would invoke executive privilege to block Bolton’s testimony, Trump said, “Well, I think you have to for the sake of the office.”

McConnell, who has been working closely with the White House on strategy, said Friday afternoon that the Senate is “anxious to get started.”

Republicans have the leverage, with a slim 53-47 Senate majority, if McConnell can keep GOP senators on board with his strategy. So far, they are supportive of modeling the trial after the one used in the last presidential impeachment, of Bill Clinton, 20 years ago. It set out a path for starting the trial and voting on witnesses later.

Despite McConnell’s wishes for a speedy trial, some Republicans in his caucus have indicated that they are open to witnesses. It takes just 51 senators to set the rules, and Democrats have been trying to win over wavering GOP senators to vote with them on hearing new testimony.

“I am hopeful that we can reach an agreement on how to proceed with the trial that will allow the opportunity for witnesses for both the House managers and the President’s counsel if they choose to do so,” Collins said. “It is important that both sides be treated fairly.”

Since the House vote on Dec. 18 to impeach the president, the showdown between Pelosi and McConnell, the two power centers in Congress, has consumed Capitol Hill and scrambled the political dynamics.

The speaker declined to send the articles to the Senate until she knew there would be a fair trial with witness testimony. She also asked McConnell for details on the trial structure she could decide who to appoint as impeachment managers. McConnell rebuffed all of her demands.

On Friday, Pelosi ended the stalemate by saying she had asked House Judiciary Committee Chairman Jerrold Nadler to be prepared to bring to the floor next week a resolution to appoint managers and transmit the articles of impeachment to the Senate. She did not announce a date for the House vote.

McConnell indicated Friday the trial would start soon. “We’ll get about it as soon as we can,” he said.

Transmittal of the documents and naming of House impeachment managers are the next steps needed to start the Senate trial. Yet questions remain in the Senate on the scope, format and duration.

Senate Minority Leader Chuck Schumer is eager to test Senate Republicans, especially those like Collins who are up for re-election in 2020, with votes to compel testimony from Bolton, acting White House chief of staff Mick Mulvaney and others who have so far resisted appearing before Congress.

“Senate Democrats are ready for the trial to begin and will do everything we can to see that the truth comes out,” Schumer said.

Bolton, who was present for several of the internal White House discussions about Ukraine policy that were at the heart of the Democrats’ impeachment case, is among the most compelling of four witnesses suggested by Schumer.

The former national security adviser clashed with the president’s Ukraine policy, saying he didn’t want to be part of any “drug deal” being cooked up. He called Trump’s personal lawyer Rudy Giuliani, who others have testified was orchestrating an alternative foreign policy outside of official channels, a “grenade” that was going to go off.

Chuck Cooper, an attorney for Bolton, declined to comment.

The House impeached Trump in December on the charge that he abused the power of his office by pressuring Ukraine’s new leader to investigate Democrats, using as leverage $400 million in military assistance for the U.S. ally as it counters Russia at its border. Trump insists he did nothing wrong, but his defiance of the House Democrats’ investigation led to an additional charge of obstruction of Congress.

On a July telephone call with Ukraine’s new president, Trump asked his counterpart to open an investigation into Biden, who is running for the Democratic Party’s presidential nomination, and his son Hunter while holding up military aid for Ukraine. A Ukrainian gas company had hired Hunter Biden when his father was vice president and the Obama administration’s point man on Ukraine. There is no evidence of wrongdoing by either Biden.

It’s still unclear who Pelosi will appoint as impeachment managers to prosecute the case in the Senate.

Nadler, D-N.Y., and House Intelligence Committee Chairman Adam Schiff, D-Calif., will most likely lead the team.

What was more certain is that the group will be more diverse than the 1999 team in Clinton’s trial, who were all male and white. Pelosi is expected to ensure the managers are diverse in gender and race, and also geographically.